for anyone who cares, I've pretty much finished my code for this project. I may go back and tweak the range of the scales, and add a button that has to be pressed to allow audio out (right now it's a drone instrument).
(Nearly) finished product: http://pastebin.com/1KMmLjC0
The LCD PWM is a problem that I'm going to solve with a separate AVR chip... it's a pretty cheap fix, and it's something I want to try anyway. That coupled with the headaches I foresee figuring out timers, and the fact that I'm already getting strange audio effects as my code grows (which sound cool at this point, but much more could ruin the instrument), I don't want this to get much larger.
Right now the LCD displays which scale is selected, and the note that's currently playing. This helps because without the LCD readout it's really hard to know where the hell you are and what you're playing. I want to be able to actually use this thing.